Research On The Regulating Vertical Distribution Of Active Layer Morphology And Photovoltaic Performance Of Organic Solar Cells

Organic solar cells(OSCs)have received a considerable amount of attention due to their light weight,low cost,and flexible fabrication.The prevailing process of OSCs active layer preparation was divided into two major categories:(1)one-step deposition and(2)sequential deposition(SD).The active layer of bulk heterojunction(BHJ)OSCs was prepared by one-step deposition of co-mingled solution(mixture of donor and acceptor materials),and the power conversion efficiency(PCE)of single-junction BHJ devices have so far exceeded 18%,benefiting from the innovation of new photovoltaics,interface materials,and device structures.The active layer morphology of the BHJ directly affected the exciton separation and charge transport processes and was a pivotal parameter in determining the device’s performance.However,morphological evolution was more complicated when preparing large-area devices.Therefore,controlling the morphology of the BHJ active layer will remain a major challenge for the commercialization of OSCs.Compared to BHJ,pseudo-planar heterojunction(PPHJ)OSCs prepared by SD reduced the complexity of donor(D)and acceptor(A)material interactions.More conducive to the formation of active layer morphology with well-defined vertical phase separation and the preparation of large-area devices later on,meanwhile,boasting better device stability as compared to BHJ devices.Therefore,the PPHJ OSCs have the potential to become a hot spot for future development.Firstly,we investigated the widely used PM6:Y6 system by employing the binary additive strategy to modulate the film morphology of the PM6 and the Y6,respectively.The analysis of the test results revealed that the addition of solid additive 1,10-koine glycol(DDO)improved the crystallinity of PM6 films,and the addition of solvent additive 1-chloronaphthalene(CN)induced a more ordered molecular crystallographic orientation of Y6,and the PM6+DDO/Y6+CN OSC device prepared by SD achieved an outstanding PCE(16.9%).In addition,ultraviolet-visible absorption(UV-vis)and dynamic X-ray photoelectron spectroscopy(DXPS)measurements revealed that the enhanced crystallinity of PM6 effectively inhibited both the effusion of the upper solvent layer into the lower film and the excessive penetration of the acceptor into the donor film during the SD process.Therefore,for the active layer morphology of the PM6+DDO/Y6+CN,the Y6 was more inclined to aggregate on the surface of the active layer,while the PM6 converged at the bottom of the active layer,forming an active layer morphology with good vertical phase separation,which improved the exciton separation efficiency and charge transport property.In this work,a suitable additive strategy was implemented to achieve a good vertical phase separation of active layer morphology and superior device performance although the same solvent,chloroform(CF),was used as the processing solvent.Therefore,a suitable additive strategy combined with the construction of PPHJ structures can have the potential to further improve the efficiency of OSCs.Secondly,we have combined our experience in spin coating with the application of SD to the blade-coating(BC)process,using the PM6:BTP-e C9 system and non-halogen solvent toluene(TL)to prepare efficient large-area devices.We precisely controlled the active layer morphology distribution in the preparation of small-area devices by the substrate temperature(ST)regulation strategy,constructing the PPHJ device with the morphology of vertical phase separation when the ST was 60°C.The highest PCE of 16.84%was achieved among the non-halogen solvent-treated BC small-area devices.In addition,the film-forming kinetic process was controlled by a suitable ST,which precisely regulated the aggregation behavior of the material.The result shows that the suppression of excessive material aggregation effectively enhanced the large-area film homogeneity,leading to a reduction in photovoltaic losses(from 69.45%to 8.48%)from small-area(0.04 cm~2)to large-area(2 cm~2)devices.This investigation established a link between film formation kinetics,aggregation of materials,and film homogeneity.The preparation of active layer morphology with good vertical phase separation was an essential parameter in enhancing device performance,and the development and design of a device structure and printing technology that facilitate large-area production is of great importance for future commercialization.